20160220

Astronomy quiz question: Mars' retrograde motion

[Version 1]
The locations of Venus, Earth and Mars are shown in the diagram below (not to scale, and orbits have been simplified as circles instead of ellipses).

Mars is __________ retrograde motion.
(A) about to start.
(B) in the middle of.
(C) just completing.

Correct answer (highlight to unhide): (C)

Retrograde motion occurs for Mars when the sun, Earth, and Mars are aligned (Mars in opposition). Drawing in lines from the sun to Earth and the sun to Mars indicates that Earth (which travels faster along its inner orbit), has already overtaken Mars (which travels slower along its outer orbit), thus Mars has already completed its retrograde motion.

[Version 2]
The locations of Venus, Earth and Mars are shown in the diagram below (not to scale, and orbits have been simplified as circles instead of ellipses).

Mars is __________ retrograde motion.
(A) about to start.
(B) in the middle of.
(C) just completing.

Correct answer (highlight to unhide): (A)

Retrograde motion occurs for Mars when the sun, Earth, and Mars are aligned (Mars in opposition). Drawing in lines from the sun to Earth and the sun to Mars indicates that Earth (which travels faster along its inner orbit), is about to overtake Mars (which travels slower along its outer orbit), thus Mars is about to start its retrograde motion.